In a typical conventional forming method of solder bumps, solder paste is applied on pad electrodes of a substrate by screen printing or a dispensing method, and the solder paste is heated for reflow. The solder paste is also referred to as “cream solder”.
Patent Literature 1 discloses an example of the above-described solder paste. The solder paste depicted in Patent Literature 1 is obtained by forming an oxide film on the surfaces of solder particles by making the solder particles flow in the air. It is noted that the oxide film formed forcibly functions to suppress coalescence of the solder particles against the effect of flux at the time of reflow. Thus, it is understood that when performing reflow after applying the solder paste all over the substrate, a solder bridge between the pad electrodes is not easily generated. Therefore, it is suitable for enabling densification and micronization of the pad electrodes. The solder bridge between the pad electrodes is a phenomenon that occurs when the solder particles are united with each other to form a large lump, and come in contact with both of the adjacent pad electrodes.
A reflow apparatus is used in a reflow process. As the reflow apparatus, there is known a type in which a substrate is placed directly on a panel heater for heating the substrate by conduction of the heat from the panel heater (first conventional example). However, this reflow apparatus has such a shortcoming that heat distribution of the substrate becomes non-uniform with a slight warp or unevenness of the substrate. There is also known a reflow apparatus in which a space is provided between the panel heater and the substrate for heating the substrate by thermal radiation from the panel heater (second conventional example). However, this reflow apparatus has such a shortcoming that the heating power thereof is insufficient. There has been developed a reflow apparatus (third conventional example, e.g. Patent Literature 2 mentioned below) that heats the substrate by applying hot air thereto for overcoming the shortcomings of the first and second conventional examples. With this reflow apparatus, a space is provided between a hot-air ejection nozzle and the substrate for heating the substrate by blowing hot air from the top and bottom part of the substrate. Therefore, the substrate can be heated uniformly and a sufficient heating power can be obtained as well.    Patent Literature 1: Japanese Unexamined Patent Publication 2000-94179    Patent Literature 2: Japanese Unexamined Patent Publication H5-92257
However, the conventional solder bump forming method has following shortcomings.
It has become difficult with the screen printing and dispensing method to cope with more densification, micronization and increased number of electrodes these days. That is, the screen printing needs to micronize the opening of a metal mask, which causes such issues that the mechanical strength of the metal mask is deteriorated and it becomes hard for the solder paste to be ejected from the opening of the metal mask. In the dispensing method, the solder paste is applied on a large number of pad electrodes one by one. Thus, it becomes unsuitable for mass production as the number of pad electrodes increases.
In the meantime, it is necessary with the solder paste of Patent Literature 1 to form the thickness of the oxide film of the solder particles with a fine precision. The reason is that the solder is not wetted over the pad electrode when the film is too thick, and the solder particles are united with each other when it is too thin. Moreover, the effect of the flux changes in accordance with the state or the kinds of the flux, so that it is also necessary because of these reasons to control the thickness of the oxide film with a fine precision. Meanwhile, densification and micronization of the pad electrodes cannot be achieved without forming the oxide film in a proper film thickness. Thus, even though the solder paste of Patent Literature 1 can be applied solid without requiring any precision mask, it is still difficult to meet a demand for densification and micronization of these days.
Further, with the reflow apparatus of the third conventional example, there have been cases where the solder bumps cannot be formed due to oxidation of the solder paste on the substrate. This is considered because many heated oxygen molecules come in contact with the surface of the solder paste because hot air is used therewith. Furthermore, there is also such a shortcoming that the quality of the solder bumps becomes unstable by the effect of the hot air when the hot air is directly blown to the fine electrodes for forming the solder bumps. Further, the particles may attach to the solder bumps by being blown to the substrate with the hot air. This can also be caused when the hot air is blown only from the bottom part of the substrate, since the hot air beneath the substrate moves around towards the above the substrate.
An object of the present invention is to provide solder bump forming method and apparatus, which can meet the demands for densification and micronization of solder bumps of these days. Furthermore, it is an object of the present invention to provide a heater, a reflow apparatus and the like with which: oxidation of the solder paste can be suppressed even when heat is applied by hot air; the quality can be made stable; and the effect of the particles is small A still further object of the present invention is to provide a heater, a reflow apparatus and the like with which the solder bumps can be formed by a liquid solder composition even when heat is applied by hot air.